Metal oxide containing coatings



' March 21, 1961 R. E. WHITE ETAL 2,976,166

4 METAL OXIDE CONTAINING COATINGS Filed May 5, 1958 2 Sheets-Sheet 1 CUR V5 2 15 0 AVAILABLE HEAT- K CALOR/fi? Q 2000 a 3000 g kg) 5 0% 4000 53 5 3 (v;

TEMPERATURE- K.

OXYGEN, '0;-

POWDERED g b-m4 i INVENTORS I o 0x105. T WIT/05mm EMezrceau/ coou/ve WATER n RobertE. White ATTORNEYS March 21, 1961 R. E. WHITE ETAL 2,976,166

I METAL OXIDE CONTAINING COATINGS Filed May 5, 1958 2 Sheets-Sheet 2 f f 0xm/v- Q) POWDERED METAL AND POWDERED M5741. (IX/0E I N VENTORS Wilh'am EMwraeaw al' 'RofiertE. Wlw'fie ATTORNEYS UeitsdStato P ten METAL OXIDE CONTAINING COATINGS Robert E. White, 14 Annabella Ave., Havertown, Pa., and William E. Marceau, 253 Township Line, Upper Darby, Pa.

Filed May 5, 1958, Ser. No. 732,981

7 Claims. (Cl. 117-23) This invention relates to coatings containing a metaloxide and, more particularly, to a method of hot-spraying metal oxide-containing ceramic and cermet coatings using anoxy-metal flame.

Flame sprayed coatings of 'the prior art have ranged from pure or substantially pure metallic coatings, on the one hand, to coatings consisting esentially of high melting metal oxides. Coatings have also been made which contain both metal and metal oxide particles. These are known, respectively, as metallic, ceramic and cermet coatings.

In general, theseare made by melting and atomizing the metal oxide or a mixture 'of metal and metal oxide by means of the heat from a flame of a burning fuel such as an oxy-acetylene or an oxy-hydrogen flame. The material to be sprayed may be in the form of powders or may have been formed into rods or wires which are fed to the flame to melt or partially melt the coating material. The velocity of the molten particles is further increased by a jet of high velocity gases such as the combustion gases of the flame, or high velocity air to propel the material while still in the molten or partially molten stage against the surface to be coated.

While various refinements of flame spraying techniques are known to the art, the flame spraying has been based essentially upon the utilization of heat of combustion of a fuel such as hydrogen or a hydrocarbon to effect melting of the metal and/or metal oxide particles, and to impart a high velocity to the molten particles for impingement on the surface to be coated while still in the molten state, thereby effecting adhesion to the substrate.

Metal oxidecontaining coatings demonstrate excellent resistance to heat, and abrasion, and, further, embody excellent thermal shock characteristics. Metal oxides melting above 1000 C., therefore, are ideally suited as coatings for construction materials, the surfaces of which are subjected to high temperatures, severe oxidizing conditions or extreme erosive or abrasive conditions. While metal oxide coatings have been known in the art for many years, the advent of supersonic aircraft, jet engines and rocket propelled missiles has increased greatly the possible field of utility for these coatings with their deshable characteristics;

The formation of ceramic coatings and some of their uses are described in United States Patent 2,707,691. Cermet coatings and some of their uses are described in United States Patent 2,775,531.

The comparatively slow rate of conductive and con- I vective heat transfer across the gas-solid interface between a burning fuel gas and the solid particle severely limits the rate at which metal oxide-containing coatings can be applied to the surfaces by the flame spraying methods previously known.

coating, but leave the system carrying a large portion of the heat of combustion hence the'thermal eflioiency of Additionally, the. combustion products of the fuel form no part of the ultimate such flame spraying methods is low, often approximating no more than 5%.. Consequently, while, intheory,

the field of application of ceramics and cermets is not limited by technological consideration, the slow rates of application and the low thermal efliciency with their resulting high labor and material costs impose economic restriction which have limited the use of metal oxide-- A containing flame sprayed coatings to physically small products, or to high value structural members where cost is not a primary factor. High melting point oxides cannot'be satisfactorily laid down as hard adherent coat ings in many of the prior art processes due to insuflicient heat available at the necessary temperature level.

Accordingly, it is a primary object of the present inven-.

tion to .provide a method for spraying metal oxidecontaining coatings which will permit high rates of application and high thermal efficiency, resulting in substantially improved economics in. application of these types of coatings over processes heretofore known. it is also.

an object to successfully deposit higher melting oxides as hard adherent coatings. Thisis made possible by the larger quantities of heat available at or near the oxymetal flame temperatures. As a result, the present invention provides a method for hot-spraying metal oxidecontaining coatings which, because of its economy, per-.

mits a wider application of such coatings.

In accordance with the present invention, there is pro videda'method for spraying metal oxide-containing coatings which comprises forming a jet of a mixture of finely divided metal and a refractory seed material such as re-,

fractory metal oxide particles and a gas containing elemental oxygen gas sufficient to burn at least a part of the metal, igniting the mixture to form a burning metal flame, and impinging the flame on a surface to be coated, an important proportion of the total heat requirement being supplied by the burning of the metal particles.

In another broad embodiment, this invention comprises forming a mixture of a powdered refractory metal oxide or other refractory seed material, a powdered metal ,7 which, upon burning with oxygen is converted to av refractory metal oxide, and a gas containing free oxygen in an amount sufiicient to burn at least a partofsaid metal to produce a high temperature flame, projecting the mixture as a jet at a velocity in excess of the flame velocity of the burning metal, igniting the jet in an igniting zone in which is maintained a means such as a pilot flame to supply heat requisite to maintain and propagate the flame of burning metal, and projecting the resultant stream containing the seed material on which are condensed metal oxide vapors produced by the burned The method of this invention utilizes the heat provided' by the flame of a burning metal to heat the'seedfparticles, I

frequently to the'sintering or fusion point, or a point at which the particles are partially butv not necessarily completely molten. A pilot flame, for example, burning hydrogen or acetylene or other fuel gas, or other suitable means such as a radiant heater, electricspark, electric arc, etc., is used to ignite the metal initially and to maintain the resultant metal flame. The heat supplied by the j pilot flame or 'other'igniting device may 'be negligible insofar as the coating'process itself is concerned, .itsf, function being 'to ignite the metal-oxygen mixture and to prevent its being extinguished as, for example, by

currents, sudden movement of the torch, etc.

. An attempt was made to form a coating b ing Patented Mar. 21, 1961 by the heat produced from said burning the flame of a burning metal such as aluminum, on a cleaned and roughened metal test piece. The result was deposition of a thin film of oxides, hardly more than a fog on the piece, and with practically no adherence thereto. Various attempts were made to condense and coalesce the metal vapors into droplets which upon striking the target would adhere, but these were unsuccessful.

We struck upon the idea of mixing particles of What we term seed material with the metal particles. These included particles of an oxide of the same metal being burned, although other materials can be used if certain requirements are met. It was found that adherent coatings were formed in which the seed material, as well as oxide from the burning metal, was a component.

We believe that upon oxidation, the metal oxide formed in burning the metal vaporizes. The seed material has not reached the temperature of the burning particles, so that the vaporous metal oxide tends to condense and deposit on the seed or nuclei of metal oxide or other inert refractory particles present in the original mixture. The intimacy of the mixture of burning metal and the seed particles, the fact that there are no significant amounts of inert gaseous combustion products in the flame (as there are in conventional flame spraying), and the phenomena of condensation of the vaporized metal oxide upon the seed particles originally present, results in an improved rate and efliciency of heat transfer. Consequently, there occurs a rapid heating of the surfaces of seed particles to a temperature below that of the oxide vapors when they form at the moment of burning. The rate of coating is thus increased enormously, frequently being 100 times faster than coating by the oxy-acetylene or oxy-hydrogen method. For example, in the case of alumina coatings, feed rates of the mixed solids of 2.0 pounds per minute or even higher may be obtained as compared with 0.02 pound per minute for a comparable gun operating with alumina rod fed into an oxy-acetylene flame according to the general teachings of United States Patent 2,707,691. The reasons for the improvements obtained are not fully understood, but we believe that they may be explained at least in part by the following considerations.

The oxy-metal flame, such as the oxy-aluminum flame, of this invention differs from the oxy-acetylene flame of the prior art, much as steam differs from air. If one pound of steam at atmospheric pressure and 213 F. and an equal weight of air at the same temperature and pressure are considered, the removal of a small amount of heat from each will reduce their temperature to 212 F. However, further removal of heat has a different effect on each. If 25 B.t.u.s are removed from each, the air temperature will be reduced to 110 F. whereas the steam will remain at 212 F. and approximately 0.025 pound will have condensed to water at 212 F. Once 970 B.t.u.s have been removed from the steam, it will have all condensed at 212 F. Additional heat removal will reduce the condensate temperature at the rate of 1 F. per B.t.u. removed. In other words, from the steam system there is available at 212 F. a large amount of heat whereas practically-none is available at 212 F. from the air system.

If a cold solid mass is placed in the steam, some of the steam will condense immediately and rapidly on the solid and the temperature of the solid will quickly approach 212 P. On the other hand, if the same cold solid mass were placed in the hot air, it would warm up slowly and the air temperature would fall below 212 F., the extent depending upon the mass and initial temperature of the solid. As an example, if the solid were a steel plate at 70 F. and weighing 1.0 pound, 0.014 pound of steam would condense on the plate, the steam temperature would remain at 212 F. and the plate would reach approximately 212 F. By comparison, in air the platewould be heated and theair cooled to approximately F. The rate of heating of the plate would be very slow compared to the rate obtained in the steam.

If the pound of air and the pound of steam were combined and the steel plate placed in the mixture, the plate would heat slowly and the mixture would cool slowly until both reached about 197 F. However, no condensation would result. The air-steam mixture must be cooled to F. before condensation would begin. Condensation could only be continued by lowering the temperature further and further. Thus, with a steam-air mixture condensation does not occur at 212 F. but begins at some temperature below 212 F. and continues over a temperature range.

When a fuel gas, such as acetylene, is pre-mixed with oxygen and the mixture combusted as it issues from the tip of a gas burner, a flame results. The flame represents the region of rapid chemical reaction resulting in practically complete combustion of the fuel gas. The composition and temperature varies considerably within the flame. The flame is usually comprised of two parts: a sharply defined bright inner cone and a less sharply defined and less bright outer envelope. Ignition occurs at the inner surface of the inner cone. As the gases progress through the inner cone and then through the outer cone, decomposition and oxidation proceed.

The local composition and temperature at various points in the flame vary considerably. The highest local temperature usually exists near the tip of the inner cone and for oxy-acetylene is about 3100 C. Decomposition of the acetylene occurs primarily in the inner cone and these products are partially oxidized before passing to the outer envelope.

The reaction of carbon with oxygen to give CO and of hydrogen with oxygen to give water, cannot go to completion at the high temperatures existing in the inner cone. A considerable amount of CO and hydrogen (atomic and molecular) as well as other decomposition products (radicals) exist at the outer surface of the inner cone. As these materials pass through the progressively cooler regions of the outer envelope, the oxidation to CO and H 0 proceeds practically to completion. The heat released in the combustion of acetylene depends upon the degree of completion of the reaction and thus upon local temperature. Therefore, only a portion of the heat of combustion has been released at the hottest part of the flame and the balance is released incrementally in the cooler regions of the outer envelope--see Fig. 1, curve 1.

In the case of a gaseous fuel, the heat released by combustion exists as sensible heat in the gaseous combustion products and as it is removed, the temperature of the combustion products is lowered. Also, because the products are gaseous, the heat can be transferred to a cold solid only by radiation or by convective heat transfer. Radiation is of significance only at the highest temperatures. Convective heat transfer to a solid from a gas is poor compared to the transfer from a liquid, or to the transfer when a vapor-liquid phase change is involved, such as in the condensation of a vapor.

Since the heat of combustion of a gaseous fuel, such as acetylene, is not available at the maximum flame temperature but becomes available by decreasing the temperature of the combustion products, the driving force for the transfer of heat to an isothermal heat sink is cor1- tinually decreasing as the heat becomes available. This is an important consideration when a gas flame is used as a source of heat.

If an excess of oxygen is supplied to the oxy-acetylene flame, the maximum flame temperature and the average flame temperature are reduced since this additional matter must be raised to flame temperature, thus holding some of the heat released. The additional oxygen also has some effect on the degree of completion of the reactions leading to CO and H 0 at the various temperatures oxygen is supplied as air, the accompanying nitrogen also lowers the maximum and average flame temperatures through absorption of heat.

If an inert, finely divided, solid material is injected into the oxy-acetylene flame, it will absorb heat by radiation and convection as it passes through the flame. The rate of heating will be a function of the temperature difference between the solid and the combustion products at any particular point along the path of the solid. It will also depend upon the thermal properties of the solid, the turbulence in the flame and the residence time of the solid in the flame. Here again the flame temperature will be lowered, the extent depending upon the amount of solid injected.

In the case of the oxy-metal flame, if finely divided aluminum is suspended in a stream of oxygen, fed through a suitable burner, and ignited as it issues from the tip of the burner, the aluminum will burn with a hot bright flame. The flame has a bright central inner zone surrounded by a cooler, less bright envelope. The envelope probably comprises air that is drawn into the flame, and aluminum oxide. Issuing from the outer envelope is a heavy smoke of finely divided aluminum oxide particles suspended in hot air.

The mechanism of the burning of aluminum powder is not clearly understood but it definitely differs from the burning of a fuel gas. It is believed that the aluminum must be heated and vaporized before it will burn with a flame. This probably occurs in the inner cone.

The combustion of aluminum may be represented as The oxide produced has a melting point of 2323 K. and a boiling point of 3800 K. The heat released by the oxidation is more than suflicient to yield the combustion product at 3800 K. Thus, a part of the oxide is vaporized. However, it is believed that, upon vaporization, the oxide decomposes according to the reaction A1 0 (liq.) 2AlO (gas)+0 (gas) (2) The excess heat of combustion is used to carry out this reaction and, thus, the maximum temperature reached is the boiling point of A1 0 The heat required to bring about Reaction 2 may be considered to be made up of the heat of vaporization of the A1 0 plus the heat of decomposition of A1 0 However, since the vaporization and decomposition occur at the same temperature, most of the heat involved in Reaction 2 can be released from the system at or near 3800 K, by reversal of the reaction, i.e., upon recombination and condensation. Therefore, the sum of the heat of vaporization and the heat of decomposition may be viewed as latent heat. This is somewhat over-simplified since the recombination and condensation will not occur at a single temperature but over a range depending upon the equilibrium relationships andthe kinetics of the reaction. Suflicient data for the reaction are not available to permit a rigorous calculation but there is evidence that very little of A exists much below 3800 K.

Based upon the above analysis of the oxy aluminum flame, the heat available at various temperature levels can be estimated. This has been done, assuming that all of the A1 0 has been re-formed and condensed at'3800 K. The results are shown in Fig. 1, curve 2. The incremental quantities of heat released below 3800 K. are from the cooling and subsequent freezing at'2323 K. of the liquid A1 0 If a finely divided nonreactive solid (seed particles) such as aluminum oxide, is injected into the oxy-aluminum flame, the particles can absorb heat. As the particles pass through the aluminum oxide vapor zone of the flame, heat and mass transfer to the cooler seed particles will take'place. A'luminum'oxide will condenseon their surfaces to an extent dependent upon their heatabsorb ing capacity. The hot liquid oxide coating will also radiate heat to the surroundings, thus permitting additional condensation to take place. The heat transfer rate can.be expected to-be high under these conditions.-

Since the condensation occurs at or near 3800' K., the

particle, beneath its coating of liquid oxide, will tend to approach this temperature with partial melting. As a result, the particle is ejected from the flame with a considerable heat content and will retain its molten coating for some distance from the flame.

As indicated above, a major part of the combustion products of the oxy-aluminum flame exist as a condensable vapor in the hottest region of the flame. If these vapors are diluted with an inert gaseous material (non condensable), the mixture must be cooled below the condensation temperature (3800 K.) of aluminum oxide before condensation will result. This is similar to an air-water vapor mixture which must be cooled to the dew point before the water will begin to condense as previously referred to. This effectively lowers the temperature at which the latent heat of condensation is released and the condensation will take place over a temperature range. Also, upon condensation, a fog is produced containing droplets of oxide suspended in a saturated gas-oxide mixture. If the dilution of the oxide is great, condensation will not occur before the freezing point is reached, and, in this case, the oxide passes from vapor state to solid state directly.

If the aluminum vaporizes prior to combustion, a diluent gas injected into the flame might be expected to dilute the aluminum vapor and alter the flame characteristics more markedly than if only the aluminum oxide vapor were diluted. The rate of combustion would be greatly reduced because of the reduced oxygen and aluminum concentrations and the flame temperature would be decreaesed. The aluminum oxide might still be produced in the vapor form, depending upon the relative amount of dilution gas, in which case it would also be diluted and behave the same as if it were diluted after formation.

If both a finely divided inert solid and an inert gas were supplied to the oxy-aluminum flame, the tendency should be to reduce the rate of heat and mass transfer to the solid particles. Therefore, the amount of oxide condensed on a particle and the heating of the particle as it passes through the flame would be reduced as the amount of inert gas is increased.

When a combination of a fuel gas, such as acetylene, oxygen (or air), and aluminum and inert solid is fed through a burner and ignited, a complex condition can be expected to exist. Depending upon the relative quantities of acetylene, oxygen and aluminum, the flame can be considered an oxy-acetylene flame or an oxy-aluminum flame.

Although a completely rigorous calculation of the flame conditions cannot be made because of insuflicient data, fair estimates can be'made through the aid of certain simplifications. This has been done for the oxy- 4 acetylene flame, the oxy-aluminum flame and two oxyacetylene-aluminum flames. The heats available at various temperatures have been calculated for -stoichiometric flames. The dissociation of CO and H 0, the combustion products of acetylene, according to the following The additional effects of Reaction 6.0m Reactions 3:. and 4 were not considered because of the complexity of the calculations.

7 Since the combustion products of acetylene and acetylene itself are, in eifect, inert diluents for the oxy-aluminum flame, they should have the efiects indicated earlier.

The results are shown in Figure 1 as the heat available versus temperature from the combustion of 1 mol of acetylene alone, 1 mol of acetylene with 0.2 mol aluminum and with 0.50 mol of aluminum, respectively, and for the combustion of 1 mol of aluminum. As far as the alumina coating processes are concerned, only heat available above the melting point of alumina is of value.

Because of the simplifying assumptions, the actual available heat at any temperature from the acetylene and the acetylene-aluminum flames, is lower than that shown. Similarly, the actual maximum acetylene-aluminum flame temperatures are lower than indicated. The calculated data for the oxy-aluminum flame is also somewhat above actual, particularly in the 3600 to 3800 K. range, since some of the oxide may still be in the vapor state.

If we consider an operation in which there is approximately 0.25 mol of aluminum per mol of acetylene, it can be seen from Figure 1 that this amount of aluminum can raise the oxy-acetylene maximum flame temperature about 100 C. if only the stoichiometric amount of oxygen is supplied (as pure oxygen). If excess oxygen, or oxygen as air, is supplied, the flame temperature increase will be much less. If a considerable amount of air as well as excess oxygen is supplied, the addition of the aluminum can have only a small effect on the flame temperature. For similar reasons, the actual temperature of the flame would be far below 3400 K. if no aluminum were added. For example, if 100% excess or any equal weight of N were supplied, the acetylene flame temperature would be reduced about 100 C.

It can also be seen from the plot that at 2323" K. about 16% of the heat available comes from the combustion of aluminum when the above acetylene-aluminum ratio and a stoichiometric amount of oxygen is supplied.

Figure 1 also shows that a large part of the heat produced from the combustion of aluminum (in the absence of inert gases) with the stoichiometric oxygen is available at or near 3800 K. This is much more significant than raising the temperature of a gaseous fuel flame a few hundred degrees. An acetylene to aluminum ratio of 1:8.7 is required if 85% of the heat available at 2323 K. is to come from the aluminum.

When aluminum is burned simultaneously with acetylene, the results are far different than the combustion of aluminum alone. When the molar ratio of acetylene to aluminum is 4:1 and stoichiometric oxygen is supplied, the composition of the flame, if the aluminum oxide is considered to be vaporized, is about 4% (volume or mol) aluminum oxide and 96% diluent (acetylene combustion products). Although data are not available from which the behavior can be precisely calculated, this extreme dilution of the aluminum oxide can have several effects. state at temperatures far below 3800 K. When condensation does occur, it takes place over a temperature range and results in fine droplets of liquid oxide. Thus, the heat from the aluminum combustion is released not at 3800" K. but at lower temperatures and over a temperature range. It seems probable that with extreme dilution all or part of the oxide passes from the vapor state directly to the solid state.

The second important effect of dilution is on the heat and mass transfer to an inert solid particle injected into the flame. When the oxide vapor is practically pure, simultaneous heat and mass transfer (condensation of oxide) to the particles occurs rapidly because of the high driving force (partial pressure of oxide) available. When the oxide vapor is diluted as with combustion products of a fuel gas, the driving force is're'duced since the partial pressure is a directfunction of molar concentration. This is believed to explain why the increased First, the oxide probably remains in the vapor.

8 coating capacities of the present invention are obtained over systems in which oxy-acetylene or oxy-hydrogen flames are used as the primary heat source.

While the prior art acknowledges the possibility of some oxidation occurring during flame spraying of metals, this is largely due to the presence of a small excess of oxidizing gas in the flame, or from contact of metal particles'with the surrounding atmospheric air. This oxidation is incidental, however, being primarily a surface effect in which a film of solid oxide may be formed directly on the surface of the metal particles. The amount of heat resulting from such oxidation is negligible compared with that produced by burning metal particles forming the flame in this invention, to provide a major proportion of the heat requirement. The burning metal particles provide more than and often about or more of the total heat requirement.

In our invention the metal oxide or other seed particles, which apparently act as nuclei upon which the metal oxide vapors produced in the flame condense, result in the formation of the relatively larger size molten oxide droplets desirable for effective coating. The proportions of seed particles in the charge may vary considerably depending upon the type of coating desired and the metal used. The presence of metal oxides in the powdered feed mixtures makes possible the retaining in the coating of the vaporized metal oxide from the burned metal, practically all of which otherwise would be lost to the atmosphere. Moreover, we believe that the liquid oxide condensed on the seed particles is responsible to a large extent for the adherence of the coating to the substrate, and for the continuity of the coating.

In the method of this invention, the mixture should contain at least about 10% by weight of metal oxide particles. In the preferred mode of operation, the mixture of metal oxides and metals fed to the flame will contain from about 30% to about 70% by weight of metal oxide particles. It appears that there should be at least 20% by weight of metal in the mixture. Excellent results have been achieved employing a mixture of about equal parts of metal and metal oxide. Since the requisite heat is derived essentially from the combustion or burning of metal particles, the mixture should contain suflicient combustible metal particles to coat the seed particles with condensed liquid metal oxide, and preferably to provide a flame temperature sufficiently high so that the oxide resulting from the burning metal is vaporous as formed. The temperature should be above the freezingpoint of the condensed oxide liquid and be maintained at this temperature until the particles strike the target. Since the adherence and physical properties of the coating, such as hardness, porosity, ctc., depends at least in part on the oxide produced from burning metal, the proportion of metal must be sufficient to give the desired effect.

The powdered mixture which provides the flame of the present invention may include particles of a wide range of sizes. While the torch may be regulated to spray powdered mixtures of any appropriate size, particularly excellent results are achieved when the seed particles range from 180 microns to about 50 microns, or even less, and the metal particle sizes range from about microns to-4 microns or less. While mixtures having a range of particle sizes, both of metal and of metal oxide, such as are normally employed by the art are excellently suited, the process of this invention likewise may be employed to spray powders which are screened or otherwise classified to a narrow specific particle size.

The particle size of the metal which is to be burned should be small. The larger the particle the longer the particle must reside in the flame region to be burned. Jet velocities at the burner tip of 300500 ft. per second have been used successfully. If the residence time is not sufficient, some of the particles either do not burn at r all or may burn only incompletely. For the ceramic gas appropriately maybe employed as the pilot gas.

coatings small metal particles are preferred. This gives a concentrated flame and a concentrated vaporand insures substantially complete burning of the metal. We have used metal particles having an average diameter of about 8 microns for producing this type of coating. This resulted in a coating which was essentially aluminum oxide.

For cermet coatings containing a high proportion of metal, the particles which are to be burned to provide the principal heat source should be as small as possible while the mixture should contain a substantial proportion of metal particles of larger sizes, for example, 50 to 180 microns. The residence time in the flame should be so regulated that the smaller particles are consumed while the larger particles either do not burn or burn incompletely.

The size of the seed particles should be larger than the particles of metal which are to be burned, particularly when making ceramic coatings, These will be larger than about 40 microns average diameter in the case of aluminum oxide. For oxides of higher density, smaller particles may be used. They should not, however, be too small since this may affect the character of the flame and the efficiency of recovery of the coating material as a coating. If they are too small even though their velocity at the burner tip is high, the momentum is low so that the very small particles may not carry to the target.

The shape of the metal particles may affect the burning characteristics. What is equally or possibly even more important, it may have an effect on the flow characteristics of the mixture in the feeding mechanism. Thus, platelets would be more difficult to feed to the torch than granules or spheres.

The shape of the seed particles has little or no effect on the coating, particularly when the seed particles become molten or partially molten in the flame. They may have an affect on the feed characteristics and, therefore, should desirably be of a character which will permit uniform feeding and transportation of the powder to the burner tip.

The metal oxide particles may be premixed with the metal particles and then mixed with oxygen prior to injection into the zone in which the burning occurs; or all three may be mixed simultaneously ahead of the igniting zone; or the oxide particles may be injected directly into the flame formed by the combustion of a mixture of oxygen and metal particles. Suitable apparatus is discussed below.

Inasmuch asit is diflicult to produce a self-sustaining flame of most metals, particularly under conditions frequently encountered in the spraying operation, we have utilized a small fuel-gas pilot flame to ignite the metal and to maintain the metal flame, i.'e., prevent its being. extinguished. Hydrogen, methane, acetylene or like fuel The rate of pilot gas is preferably maintained at close to a predetermined minimum found effective to ignite and maintain the metal flame in agiven apparatus. It is preferably located adjacent to the burner tip but externally of the oxy-metal flame itself so as to produce a minimum dilution elfect with the resultant disadvantages discussed. above. The purpose of the flame is to supply the increment of heat needed to maintain a continuously burning metal flame. device is not critical ,to the process since other ignition means may be used. The coating composition and char-.

acteristics are controlled by other means which will be discussed below, andxare not a function of the ignition device or the heat derived therefrom.

The amount of oxygen employed in the process of the present invention will depend upon the apparatus, the particlesize of the powdered feed mixture, and the desired composition of the coating. The flow of gas must have a sufficient linear velocity to suspend and.

The selection of a specific ignition inch inside diameter tip and a tip velocity of 350. ft./se c.,

the tip of the flame is about 12 inches from the torch.,

transport the powdered metal and metal oxide particles, and further must beof sufficient linear, velocity to pre vent the flame from flashing back into the burner. In other words, the velocity of the oxygen stream and suspended particles must exceed flame velocity but must not be excessively high or it will extinguish the flame or reduce the flame temperature below satisfactory operating conditions. The considerations present are analogous to any flame propagating process and are within the skill of the art. Commercially pure oxygen is preferred, and the amount used must be suificient to provide that which is required for burning the desired amount of the metal particles. A gas high in oxygen and low in inert gases is preferred since this results in a more concentrated flame and higher flame temperatures. Amounts which will provide a tip velocity of 200-600 feet per second are satisfactory when burning aluminum to form ceramic and cermet coatings from a mixture composed of equal parts aluminum and aluminum oxide.

The coating is applied by moving the torch or the article to be coated relative to each other. The surface of the article is at a relatively lower temperature than the flame. The hot coating materials impinge only briefly against a point on the surface, and hence are rapidly cooled.

For a given mixture the characteristicsof the coatings applied by the process of the present invention may be controlled by regulating the time between the beginning of burning of the metal and the time of deposition of the particles on the surface to be coated; and/ or by regulating the size of the particles; or, within limits, by regulating the amount of oxygen as will appear more fully hereinafter. The latter is not preferred.- The residence time of the particles in the flame zone may be regulated by varying the velocity of the gas medium; or by varying the distance between the surface to be coated and the tip of the torch when using a constant jet velocity; or both. Shorter distances between the relatively cooler surface to be coated and the tip of the torch will result in a lower residence time which may affect the extent of burning and may result in forming a coating containing unburned metal particles. Variation in the distance may result in a variation in the temperature at which the particles impinge on the surface with resultant variation in the character of the coating even though burning of the.

metal'is complete, as appears from the examples.

Cermet coatings, comprising mixtures of oxides and metals, may be produced by providing incomplete combustion of the metal particles. This may be done by using insuflicient oxygen to burn all of the metal'paiticles; by feeding some relatively large metal particles to the torch along with small particles so that the larger metal particles are incompletely burned before deposition; by positioning the tip of the torch'close to the surface to be coated so that there is insufficient time for complete combustion of i the metal; by increasing the velocity 'of theimpinging stream thus decreasing the residence time below that for complete combustion; by grading the size and/or shape of the metal particles to provide some which have greater surface area than others, or have 'a shape less readily ignitible than others, so that some particles burn to furnish the necessary heat, and others melt and are deposited without substantial burning; or by adding a large quantity of finely powdered metal oxidewhich will produce a wider flame 'and'thereby lower the oxygen concentration within the flame. Any one or more of these control variables may be employed to produce cermet coatings having a variety of compositions and/or properties. I

For example, when using a 50-50 mixture of aluminum particles (-8 micron) and alumina (-50 micron), 0.25

When the workis sprayed at this'distance a hard ceramic coating is produced. ,If the torch is moved about 'two inches closer, a cermet is produced from the semi-aminl. i ture. If the distance is increased say to inches a ceramic coating is produced but it is considerably softer than the first mentioned coating.

In general, any metal may be burned which has a heat of combustion sufliciently high to raise the temperature of its combustion products in the flame to a point above their melting points and preferably to or above their boiling points. Thus, any metal may be used which when burned in oxygen yields a flame temperature near or above the varporization temperature of the combustion products and whose combustion products can exist in the liquid state over a substantial temperature range and freeze at temperatures above 1000 C. In other words, the heat of combustion of the metal should be equal to or greater than the enthalpy of the combustion products at temperatures above their melting points and preferably at or above their vaporization points. Additionally, the usable metals must burn in oxygen to yield an oxide which is desirable component of the coating. The oxide produced in burning the metal should exist as a liquid over a range of at least 300 C., i.e. there should be a range of at least 300 C. between the freezing point and the boiling point of the oxide. The metal should yield, upon burning, an oxide having a melting point above 1000 C. which is condensable to the liquid state above about 1300" C.

The metal powders which may be burned include aluminum, titanium, zirconium and thorium or mixtures thereof. Mixtures of aluminum and magnesium may be used. Alloys of the foregoing metals with other metals may be used providing they have the necessary combustion characteristics described above.

Among the seed materials are alumina, zirconia, titania, thoria, magnesia, silica and the like, or mixtures thereof. Refractory oxide compounds such as silicates, etc, may be used. If desired, the process of the present invention also may be employed to spray mixtures of such a metal and a refractory oxide of a different metal.

It is important that the seed material be a desirable component of the coating; that it have no deleterious effect on adherence of coating; that it not react with the metal oxide produced by burning to produce a less desirable coating; that it melt above about 1000" C.; and that it not vaporize at the temperature it reaches in the flame. Any material meeting these requirements may be used.

The coating may be applied in a single pass or, if heavier coatings are desired, several passes may be ap plied to build them up to a desired thickness.

The process of the present invention may be employed to coat a wide variety of metallic and non-metallic surfaces. These surfaces include, inter alia, carbon steel, stainless steel, cast iron, brass, copper, aluminum, concrete, clay brick, refractory brick, insulating brick, glass and certain types of plastics. The surfaces to be coated must be cleaned and roughened prior to the application of the coating.

To prepare the surface, all grease, dirt, scale, etc. should be removed by brushing, cleaning with solvents, etc. The thoroughly cleaned surface is then grit blasted. This is done to roughen the surface which is highly important in obtaining good adherence. In general, the rougher the surface the *etter. The grit selected may depend upon the nature of the substrate. For steel, and other hard materials, angular steel grit (made by crushing steel shot) may be used, as well as fused alumina, silicon carbide or other suitable abrasive materials, in grain sizes chosen to be as coarse as possible. Grain sizes from No. No. 24 are usually suitable, but finer sizes can be used for special cases when, because of metal deformation or for other reasons, the usual sizes are considered to be too coarse.

The blast is usually directed at right angles to the work at -80 p.s.i. or higher air pressure. This develops a maximum number of sharp peaks and reentrant angles to afford maximum adherence. The work should be carefully inspected to be sure that there are no unblasted, or incompletely blasted areas.

The coating should be ployed to start and stop the flow of powder.

applied as soon as possible after blasting, preferably within a few hours, and should not be allowed to rust or become wet or dirty. Metalized undercoating with oxidation resistant metal such as nichrome tends to improve adhesion.

The apparatus employed in the practice of the present invention comprises a suitable torch provided with a pilot flame or other igniting means, and means for the pickup of metal and metal oxide particles by the burning medium and means feeding the mixture at a uniform rate to the torch. It is essential for good coating procedure that the suspension of solid particles in the oxygen stream be maintained as uniform as possible so as to avoid bursts which would momentarily shift the position of the different flame zones relative to the target and thus effect the uniformity of the coating.

Means should be provided for the introduction of pilot gas and its application at the ignition point. While the pilot gas may be premixed with oxygen or air, such procedure may be hazardous and is not preferred. The pilot gas appropriately may be introduced at the point of igni' tion of the flame, i.e. at or near the torch tip where it mixes with part of the oxygen in the incoming feed stream and is burned.

In order to provide a fuller understanding of the method of the present invention, the attached drawings illustrate an apparatus which may be employed in the practice of this invention. The invention is not limited to the particular apparatus illustrated and other suitable devices will suggest themselves to a person skilled in the art.

Figure 1 is a graph showing the results obtainable ac o ding to the method of the invention.

Figure 2 shows in diagrammatic form a suitable system for carrying out the invention.

Figure 3 illustrates a suitable torch.

Figure 4 illustrates an end view of the torch tip.

A powderfeeder which mav be employed for this invention is shown in Figure 2. Oxygen from pressure storage cylinders is metered into line 1 which is divided at point 2 into lines 3 and 4. Valve 5 in line 3 creates a pressure drop in line 3. Oxygen flow through line 4 is controlled by a pressure regulating valve 6. Downstream from valve 6. line 4 is divided into lines 7 and 8. Oxygen flows through line 7 into hopper 9 containing a uniform mixture of powdered metal and powdered seed material. It will be apparent that the powdered metal may be stored in one hopper and the powdered metal oxide in another hopper is so desired. Line 8 leads to a point ad acent the bottom of the hopper 9 and is of reduced diameter at its discharge end in order to create a high velocity gas jet. The powders are drawn into the low pressure zone of the jet and are then ejected with the oxygen into line 10. Valve 11 in line 10 is em- At point 12, the oxygen-powder stream combines with the oxygen stream flowing through line 3 and passes to the torch. A

- vibrator 13 may be employed to prevent bridging of the powder in the hopper. The powder feed rate is controlled by varying the ratio of mass flow of oxygen in lines 3 and 4, by varying the mass flow of oxygen in line 4, or by a combination of the two.

A suitable torch for producing the metal flame is shown in Figure 3. The oxygen carrying the powdered metal and metal oxide is conducted through the central tube 14 of the torch. This tube must be of such size that the oxygen velocity is sufficient to transport the powders and the discharge velocity from this tube must exceed the flame velocity to prevent flashback. The actual diameter of the tubewill be dependent upon the amount of material to be fed.v The shape of the tube and the tip may be varied as desired, depending on the shape of in the tip to reduce the diameter of the outlet. The ignite: gas, preferably hydrogen, is supplied through the annular space formed, between the inner tube 14 and a concentrically located outer tube 15. The size of thean nular space is not critical. The hydrogen leaves through the annular ring at the end of the torch.

An end view of the torch is shown in Figure 4 wherein the pilot gas emerges through concentric holes 18. Two additional concentric tubes 16 and 17 may be included to provide water cooling; however, this is not always required since uncooled torches have been found to be satisfactory.

We have found that the crystalline structure of the oxides contained in the final coating may difier considerably from that produced in accordance with prior art processes. Thus when aluminum oxide in the form of rods is applied by means of oxy-hydrogen or oxy-acetylene flame, the alumina in the coating is primarily in the gamma-type regardless of the type of the alumina in the feed. When aluminum is burned in the absence of seed particles, and the vapors condensed, the oxide is also found to be largely in the gamma form. This apparently has something to do with the rate at which the oxide solidifies and is cooled below the temperature at which the gamma form is transformed to the alpha crystalline form with any degree'of rapidity.

We have found, however, that a different situation exists in our process. When alpha alumina is mixed with metallic aluminum and burned as taught herein, the resultant coating contains alpha alumina as the predominant crystalline structure. While we are not certain as to why this should be, it appears that it could be brought about by one of the following mechanisms.

(A) An alpha alumina feed particle is only surfacecovered with molten oxide from the alumina vapor condensation.

(B) An alpha alumina feed particle does not have its original crystal structure completely destroyed in the flame but contains residual nucleation of alpha alumina type upon which the alumina vapor from the burning metal condenses and crystallizes.

(C) An alpha alumina feed particle which is completely molten impinges on an unmolten particle of (A) or the partially molten particles of (B).

Upon being quenched the molten alumina, whether from particles of the feed alumina or of the condensed vapors, tend to assume the crystal structure of the alpha alumina still present in a nucleated form. In other words, the seed material acts not only as nuclei for condensation, but also as nuclei for crystal growth. The resulting crystal is therefore not gamma alumina as might be expected but takes the form of the alpha alumina of the crystal nuclei present either in the flame or on the work surface as a result of one or all of the mechanisms outlined. Since alpha alumina is the least reactive and most heat stable of all of the crystal forms of alumina, this effect is highly desirable and constitutes an unexpected result which is not and cannot be accomplished by the prior art methods.

The following specific examples are included to demonstrate specific embodiments of the present invention. These examples are included for illustrative purposes only and are in no way intended to limit the scope of the invention.

EXAMPLE I A mixture of 50 parts by weight of aluminum metal particles having an average particle size of approximately 6 microns and 50 parts by weight of aluminum oxide particles having an average particle size of approximately 88 microns were blended together. One pound per minute of the blended particles was suspended in a stream of 150 liters per minute of commercially pure oxygen and fed through a torch similar to that illustrated in Figure 2. The linear velocity of the jet stream at the torch tip was 300 feet per second. Ten liters per min- Ute of hydrogen were provided as a pilot gas.

The torch tip was positioned at a distance of 20 inches from the steel surface to be coated. The coating was a soft, porous coating of ceramic aluminum oxide which could be scratched by the steel blade of a knife. The coating was suitable as a lining for ducts which transport high temperature, relatively slow moving gases.

EXAMPLE II The process of Example I was repeated with the torch tip positioned at a distance of 15 inches from the steel surface to be coated. The resultant ceramic aluminum oxide coating was almost entirely alpha alumina. The coating was a hard dense coating which could not be scratched with a knife. It was excellently suited for high temperature, highly abrasive service.

EXAMPLE- 111 The process of Example I was repeated with the torch tip positioned at a distance of 10 inches from the steel surface to be coated. This produced'a cermet coating which contained over aluminum oxide, the remainder being aluminum.

EXAMPLE IV EXAMPLE V The method of Example IV was repeated employing a mixture of 20% aluminum-and 80% aluminum oxide... The aluminum supplied approximately 96% of the total heat released. The coatings were relatively soft and consisted of aluminum oxide. This is the minimum metal content which can be used to produce adherent coatings.

The adherence and hardness of the coating is improved at 25% and still further at 30% aluminum.

EXAMPLE VI A series of metal plates were coated using a variety of feed mixtures containing metal, and metal oxides as the seed material. The results of these tests are summarized in Table I. The plates were prepared by cleaning and grit-blasting, in a manner previously described, using air pressures of 4080 psi. The rates of feed to the torch was about one pound per minute, except in run 3 where it was about 0.8 pound per minute, and in runs 9 and l0, where it was about 0.5 pound per minute. In 1 the latter two runs the tip velocity was about 350 feet per second and, in the other cases, was about 550 linear feet per second. Commercial oxygen was used, being fed at the rate of about liters per minute. The pilot flame was supplied with about ten liters per minute of hydrogen. The coating distance, i.e., the distance between the tip of the torch and the target was about 12-14 inches, with the exception of run 3 in which the tip was approximately 16 inches from the sample.

The results indicate'that when using the most finely divided aluminum powder the coatings produced are es?"- sentially ceramic, containing little or no metallic aluminum. The values of less than 0.4% aluminum are indi: cated b-ut quantities of aluminum of less than this value could not be detected by the test procedure employed; The actual values for aluminumin the coating are pm? sibly less than 0.1%.

When some of the coarser aluminum metal was included in the feed mixture, the coatingscontained metallic aluminum of the order of about 3.3 to 5.4%"under' the test conditions employed. Increasing the f,

content beyond about 50% resulted in the appearance of metallic aluminum in the coating under the test procedure.

The results show that mixtures of aluminum with oxides other than alumina results in a coating containing the other oxide.

One test in which metallic magnesium powder was used as the metal along with aluminum oxide yielded a soft coating which appeared to contain some metal together with alumina and magnesium oxide.

The alumina used in making these coatings was a commercial alpha alumina, 90% of which passed through a 100 mesh screen but was retained on a 325 mesh screen. The magnesium employed was minus 325 mesh. The magnesium oxide and zirconium oxides were stabilized oxides of about minus 100 mesh size. The titanium oxide was about minus 120 mesh size.

The crystallized character of the constituents was de termined by microscopic examination and X-ray examination. Where alpha alumina was included in the feed, practically all of the alumina in the coating was in the alpha form. Some other forms of alumina appeared to be present. These were poorly crystallized and ditficult to identify and are grouped mainly as gamma alumina. The proportions of the forms other than alpha,

16 seed particles, each 100 parts by weight of combined metal and seed particles comprising at least 30 parts of metal and at least parts of seed particles, said metal upon burning yielding available heat at the melting point of the metal oxide produced by the combustion of the metal, at least equal to the enthalpy of said oxide at said melting point, said available heat comprising at least 85% of the heat requisite for the coating operation, impinging the fiame against the article to form an adherent coating contain- 10 ing both the seed particles and the oxide formed in burning the metal.

2. The method of claim 1 wherein a portion but not all of said combustible metal is burned, and wherein the adherent coating contains the seed particles, the oxide 5 formed in burning the metal and the unburned metal.

3. The method of claim 1 wherein the combustion of the metal provides at least about 95% of the total heat requirement.

4. The method of claim 1 wherein said mixture contains refractory metal oxide seed particles of the same oxide produced in burning the metal.

5. The method of claim 1 wherein the solids feed mixture contains refractory metal oxide seed particles in proportions from about to about 70% by weight.

25 6. The method of claim 1 wherein the metal is alumihowever, were quite small. num.

Table I Feed Mix, Wt. Percent Coating Composition, Constituents Percent Wt.

Hard- Run Oxide Oxide Anisotropic ncss A1103 A1 X-ray A1103 Other A110; Other Amt., Size,

Percent mu 50 H 50 H r'ii 1 "1'6 "to H N ut-ie'Az 3-1). H 2 LGamnia A1103 II 0.0 MP! 0.0 M H 25 a G MEI 33% ZrOa, 33 85.5 ZrOz, l4. 20 aAlzOa Cubic ZrOz MHZ 0.0 MgO, 4 92. 6 i IgO, 7.4.. 1 vii/11206; MgO, some A1103 8 i g 0.0 MgO, 25.. 0. 4 87. 2 MgO, 12.8-. S

25 MgO, 25 0. 4 85. 3 MgO, 14.7-... MEI 50 86. 7 MgO, 5.5".-- S

Some poorly crystallized gamma A1103, mostly aAliOa.

b 5% Al based on total feed=80% through 80 mesh, remainder #900 mesh. All other tests used 900 mesh Al powder. Both were commercial aluminum powders.

s H=hard, MH=mcdium hard. S=very soft, can be scratched with a steel knife blade.

d Magnesium powder, 50% of feed.

7. The method of claim 1 wherein the metal is aluminum and the seed particles are aluminum oxide.

References Cited in the file of this patent UNITED STATES PATENTS Montgomery et al Dec. 25, 1956 FOREIGN PATENTS Great Britain Nov. 30, 1933 

1. A METHOD OF FORMING A METAL OXIDE-COATING ON AN ARTICLE WHICH COMPRISES FORMING AND MAINTAINING A FLAME FROM A COMBUSTIBLE MIXTURE, THE COMBUSTIBLE PORTION OF SAID MIXTURE CONSISTING ESSENTIALLY OF OXYGEN AND A COMBUSIBLE METAL, THE OXIDE FORMED FROM SAID METAL MELTING ABOVE 1000*C. AND BEING CONDENSIBLE TO THE LIQUID STATE ABOVE 1300*C., ENTRAINING IN SAID FLAME REFRACTORY SEED PARTICLES, EACH 100 PARTS BY WEIGHT OF COMBINED METAL AND SEED PARTICLES COMPRISING AT LEAST 30 PARTS OF METAL AND AT LEAST 10 PARTS OF SEED PARTICLES, SAID METAL UPON BURNING YIELDING AVAILABLE HEAT AT THE MELTING POINT OF THE METAL OXIDE PRODUCED BY THE COMBUSTION OF THE METAL, AT LEAST EQUAL TO THE ENHALPY OF SAID OXIDE AT SAID MELTING POINT, SAID AVAILABLE HEAT COMPRISING AT LEAST 85% OF THE HEAT REQUISITE FOR THE COATING OPERATION, IMPINGING THE FLAME AGAINST THE ARTICLE TO FORM AN ADHERENT COATING CONTAINING BOTH THE SEED PARTICLES AND THE OXIDE FORMED IN BURNING THE METAL. 